As mobile devices have been increasingly developed, and the demand for such mobile devices has increased, the demand for secondary batteries has also sharply increased. Among such secondary batteries is a lithium secondary battery exhibiting high energy density and operating voltage and excellent preservation and life span characteristics, which has been widely used as an energy source for various electronic products as well as varieties of mobile devices.
Based on the shape of a battery case, a secondary battery may be classified as a cylindrical battery having an electrode assembly mounted in a cylindrical metal container, a prismatic battery having an electrode assembly mounted in a prismatic metal container, or a pouch-shaped battery having an electrode assembly mounted in a pouch-shaped case formed of an aluminum laminate sheet. The cylindrical battery has advantages in that the cylindrical battery has relatively large capacity and is structurally stable.
The electrode assembly mounted in the battery case serves as a power generating element, having a cathode/separator/anode stack structure, which can be charged and discharged. The electrode assembly may be classified as a jelly roll type electrode assembly configured to have a structure in which a long sheet type cathode and a long sheet type anode, to which active materials are applied, are wound in a state in which a separator is disposed between the cathode and the anode, a stacked type electrode assembly configured to have a structure in which a plurality of cathodes having a predetermined size and a plurality of anodes having a predetermined size are sequentially stacked in a state in which separators are disposed respectively between the cathodes and the anodes, or a stacked/folded type electrode assembly, which is a combination of the a jelly roll type electrode assembly and the stacked type electrode assembly. The jelly roll type electrode assembly has advantages in that the jelly roll type electrode assembly is easy to manufacture and has high energy density per weight.
In connection with the above, the structure of a conventional cylindrical secondary battery is shown in FIG. 1.
Referring to FIG. 1, the cylindrical secondary battery 10 generally includes a cylindrical container 20, a jelly roll type electrode assembly 30 mounted in the container 20, a cap assembly 40 coupled to the upper end of the container 20, and a crimp region 50 at which the cap assembly 40 is mounted.
The electrode assembly 30 is configured to have a structure in which a cathode 31 and an anode 32 are wound into a jelly-roll shape in a state in which a separator 33 is disposed between the cathode 31 and the anode 32. To the cathode 31 is attached a cathode tab 34, which is connected to the cap assembly 40. To the anode 32 is attached an anode tab (not shown), which is connected to the lower end of the container 20.
Meanwhile, a lithium secondary battery has a disadvantage in that the lithium secondary battery has low safety. For example, when the battery is overcharged to approximately 4.5 V or more, a cathode active material is decomposed, dendritic growth of lithium metal occurs at an anode, and an electrolyte is decomposed. At this time, heat is generated from the battery with the result that the above-mentioned decompositions and several sub decompositions rapidly progress and, eventually, the battery may catch fire and explode.
In order to solve the above-mentioned problems, therefore, a general cylindrical secondary battery includes a current interruptive device (CID) and a safety vent mounted in a space defined between an electrode assembly and a top cap to interrupt electric current and release internal pressure when the secondary battery malfunctions.
Specifically, the cap assembly 40 includes a top cap 41 constituting a cathode terminal, a positive temperature coefficient (PTC) element 42 for interrupting electric current through the great increase of battery resistance when the interior temperature of the battery increases, a safety vent 43 for interrupting electric current and/or exhaust gas when the interior pressure of the battery increases, an insulating member 44 for electrically isolating the safety vent 43 from a cap plate 45 excluding a specific portion, and the cap plate 45 connected to the cathode tab 34, which is attached to the cathode 31. The cap assembly 40 is configured to have a structure in which the top cap 41, the PTC element 42, the safety vent 43, the insulating member 44, and the cap plate 45 are sequentially stacked.
The crimp region 50 is formed at the upper end of the container 20 such that the cap assembly 40 can be mounted to the open upper end of the container 20. More specifically, the crimp region 50 is formed by beading the upper end of the container 20 such that a beaded part 21 is formed at the inside of the container 20, sequentially inserting the outer circumferences of the cap plate 45, the insulating member 44, the safety vent 43, and the top cap 41 into a gasket 60, and bending the upper end of the container 20. As a result, the crimp region 50 is formed in a shape to surround the gasket 60 positioned at the inside of the crimp region 50. The cap assembly 40 is mounted at the crimp region 50 by crimping and pressing.
Since welding between the cathode tab 34 and the cap plate 45 is performed at the inside lower end of the gasket 60, however, interference may be caused due to the structure of the outer circumference of the lower end of the gasket. As a result, contact between the cathode tab and the cap plate may be weakened.
In addition, when the battery falls or external impact is applied to the battery, the jelly roll may rotate in a specific direction or move upward and downward with the result that a short circuit may occur at a cathode tab contact region.
Therefore, there is a high necessity for technology to fundamentally solve the above problems.